Currently, aircraft designers and manufacturers use various flexible, joints throughout the manufacture of the jet engines for use in the aircraft. To this end, various attempts to develop a satisfactory flexible, moderately pressurized joint have been made.
For example, U.S. Pat. No. 4,448,449, issued to Halling, et al. and entitled “Flexible Piping Joint and Method of Forming Same” (Halling), discloses a fluid-tight coupling and sealing apparatus. Hailing describes a flexible piping joint, for use in fluid systems at moderate pressures and temperatures, that require a limited amount of angulation during operation. The invention in Halling, however, employs structurally inefficient load paths and non-metallic sealing elements. Further, these elements are not feasible at the extreme temperatures and pressures of rocket engine applications. Moreover, the sealing interface depends on the interference fit between the non-metallic seal and the metal duct material, both of which have significantly different thermal coefficients of expansion which limit the allowable operating temperature range. Additionally, the spherical interface must react with the pressure-separating load with the hoop strength of the concentric rings through a very structurally inefficient contact angle. Thus, for high pressure applications, the ring thicknesses would be significant, resulting in a very heavy structure, much larger in diameter for a given duct diameter.
U.S. Pat. No. 4,772,033, issued to Nash and entitled “Flexible Duct Joint Utilizing Lip in Recess in a Flange” (Nash), discloses a flexural joint for connecting opposing ends of two annular ducts. The invention in Nash is directed towards large diameter thin wall jet engine casing joints that permit both torsional and transverse motion. The spherical interface possesses a much larger diameter than the casing diameter, with the retaining bolt pattern possessing an even larger diameter. However, this type of structurally inefficient interface is also only acceptable for low-pressure applications that do not have to react large separating loads. For example, in high-pressure applications, the interface seal must be positioned close to the duct internal diameter to minimize the pressure separating load. Additionally, the interface bolts must be preloaded at a stiffness level sufficient to preclude separation at the seal interface with the high operating pressures. Thus, the teachings of Nash are not applicable or structurally feasible for high-pressure applications.
Finally, U.S. Pat. No. 5,697,651, issued to Fernandes and entitled “Flexible Duct Joint Having a Low Leakage, Pressure-Balanced Bellows Seal” (Fernandes), discloses a flexible joint for sealing two conduits. Fernandes discloses a flexible duct joint for aircraft engines possessing compressed air ducting joints that operate at relatively low pressures, as compared to rocket engine joints. The joint concept permits angulation motion during operation with limited leakage which is acceptable in the compressed air system. However, Fernandes utilizes joint structural shapes, retention mechanisms and multi-convolution bellows that are not feasible for rocket engine high pressure cryogenic and hot gas systems that require zero leakage.
Although the aforementioned references do provide flexible joints to overcome jet engine operating conditions, the references nevertheless fail, in one form or another, to facilitate the much more extreme conditions that exist in a rocket engine. This is primarily due to the fact that the aforementioned references are typically conceived for applications with operating pressures less than 1000 pounds per square inch (psi). The references are, generally speaking, not structurally efficient or feasible enough for applications within the 8000 psi range.
Since the early development of liquid-fuel rocket engines, the need to transfer propellants from low pressure supply tanks to turbopumps, turbine-driven pumps that raise the propellants used therein to pressures high enough for injection into a combustion chamber, has required specialized ducting that can, inter alia, accommodate assembly misalignments, thermal induced defections and both pressure- and vibration-induced loads. Early rocket engines typically operated at combustion chamber pressures of less than 1,000 pounds per square inch (psi), which required pump discharge pressures of less than 2,000 psi. For these applications, ducting, including tied bellows and braided hoses adapted from the aircraft engine and petro-chemical industries, were utilized to accommodate the aforementioned misalignment and deflections.
However, as combustion chamber pressures were increased from less than 1,000 psi to greater than 3,000 psi and closed cycle engines were introduced, a need for propellant ducts operating at up to 8,000 psi at temperatures as low as −400° F. and hot gas ducts operating at up to 6,000 psi at temperatures as high as 1,200° F. were established. The use of tied bellows or braided hoses are not feasible at these operating conditions, so solid wall ducts possessing sufficient length and routing, and flexible enough to accommodate the deflections were utilized. The excessive weight of the complex ducting created the need for flange joints that could accommodate assembly misalignments and react to the pressure- and vibration-induced loads without leakage at the extreme operating conditions.